Process of preparing finely divided thermoplastic resins

ABSTRACT

A PROCESS FOR PREPARING FINELY DIVIDED THERMOPLASTIC RESINS SUCH AS POLYETHYLENE WHICH COMPRISES VIGOROUSLY AGITATING MOLTEN RESIN IN ADMIXTURE WITH WATER AND IN THE PRESENCE OF A BLOCK COPOLYMER OF ETHYLENE OXIDE AND PROPYLENE OXIDE TO PRODUCE A FINE DISPERSION, AND THEN COOLING THE RESULTING DISPERSION TO A TEMPERATURE BELOW THE MELTING POINT OF THE RESIN. THE RECOVERED FINELY DIVIDED THERMOPLASTIC RESINS ARE CHARACTERIZED BY A NARROW PARTICLE SIZE DISTRIBUTION AND SUBSTANTIALLY SPHERICAL SHAPES.

United States Patent 3,746,681 PROCESS OF PREPARING FINELY DIVIDEDTHERMOPLASTIC RESINS Dorothea M. McClain, Cincinnati, Ohio, assignor toNational Distillers and Chemical Corporation, New York, NY.

No Drawing. Continuation-impart of abandoned application Ser. No.775,147, Nov. 12, 1968, which is a continuation of application Ser. No.37 0,006, May 25, 1964, now Patent No. 3,422,049, which in turn is acontinuation-in-part of abandoned application Ser. No. 160,733, Dec. 20,1961. This application July 6, 1971, Ser. No. 160,194 The portion of theterm of the patent subsequent to Jan. 14, 1986, has been disclaimed Int.Cl. C08b 29/36; C08f 47/18; C08g 53/20 US. Cl. 26029.6 PM 7 ClaimsABSTRACT OF THE DISCLOSURE A process for preparing finely dividedthermoplastic resins such as polyethylene which comprises vigorouslyagitating molten resin in admixture with water and in the presence of ablock copolymer of ethylene oxide and propylene oxide to produce a finedispersion, and then cooling the resulting dispersion to a temperaturebelow the melting point of the resin. The recovered finely dividedthermoplastic resins are characterized by a narrow particle sizedistribution and substantially spherical shapes.

This invention relates to a process for making finelydivided, normallysolid, synthetic organic polymeric thermoplastic resins. It is acontinuation-in-part of US. patent application Ser. No. 775,147, filedNov. 12, 1968, now abandoned which is a continuation of US. patentapplication Ser. No. 370,006, filed May 25, 1964 and issued on Jan. 14,1969 as US. Pat. No. 3,422,049, which was a continuation-in-part of Ser.No. 160,733 filed Dec. 20, 1961, now abandoned.

Thermoplastic resins in finely-divided form have found use in a numberof applications where it is either impossible or inconvenient to utilizethe more conventional cube or pellet forms. For example, powderedorganic polymeric thermoplastic resins in dry form have been used tocoat articles by dip coating in either a static or fluidized bed, bypowder coating wherein the powder is applied by spraying or dusting, andby flame spraying. In dispersed form, thermoplastic resin powders havebeen applied as coatings by roller coating, spray coating, slushcoating, and dip coating to substrates such as metal, paper, paperboard,and the like. These powders have also been widely employed inconventional powder molding techniques. Other applications of thesepowders include paper pulp additive; mold release agent for rubber;additives to waxes, paints, and polishes; binder for non-woven fabrics;and so on.

Prior art processes for making normally solid finelydivided organicpolymeric thermoplastic resins from coarser forms such as cubes,pellets, coarse powders, and the like, which forms usually are obtaineddirectly from the synthesis process, have been concerned primarily withpolyolefins. These processes of subdivision are of three main types;mechanical grinding, solution, and dispersion.

In the first type, the polyolefin in granular form is 3,746,681 PatentedJuly 17, 1973 ice passed through a high shear pulverizing device, e.g.,a Pallmann grinder, to yield particles of irregular shape havingdiameters ranging from about to 300 microns. In addition to requiringspecially designed equipment, such processes yield powders which are notentirely suitable for fluidization or dispersion application whereinspherical particles of narrow size distribution are required.

The second type of prior art process generally entails dissolving thepolymer in a solvent, followed by precipitation of the polymer infinely-divided form through addition of a nonsolvent or evaporation ofthe solvent or a combination of the two. Emulsifying agents sometimesare employed to aid particle breakdown. Inherent in such processes arediificulties in manipulating the solvents, completing removal of thesolvent from the product and classifying the resultant powders. Thepowders from such processes are of irregular, somewhat rounded shape,and consequently possess only moderately satisfactory fluidizationcharacteristics.

The third type of prior art process involves dispersion under high shearagitation of a polymer in a liquid medium with the aid of variousdispersing agents. From: the standpoint of cost and simplicity ofoperation, water is generally the preferred dispersing medium. Thedispersing agents usually comprise a soap such as sodium stearate orsome other type salt. Processes wherein such agents are used generallyrequire all or a portion of the dispersing agent to be incorporated intothe polymer in a separate step preceding dispersion in water. In caseswhere the final product is desired in a powdered state, the presence ofdispersing agent residues in the polymer generally creates undesirablechanges of the original polymer properties, e.g., increasedwater-sensitivity, loss of electrical insulating values, and the like.Removal of such residues is generally diflicult, however, and oftenimpossible. Another disadvantage of these dispersing agents is that theytend to become inactive at temperatures below which only relatively lowmolecular weight polyolefins are sufficiently fluid to be dispersible inwater. Consequently, such prior art processes have generally beenlimited to relatively low molecular weight polyethylenes.

Accordingly, it is an object of this invention to provide a process formaking finely-divided synthetic organic polymeric thermoplastic resins.

Another object of the present invention is to provide a process formaking finely-divided polyolefins which does not require the specializedequipment or solvent combinations required by prior art processes.

A .more particular object of this invention is to prepare finely-dividedsynthetic organic polymeric thermoplastic resins by means of an aqueousdispersion process.

A still more particular object is to make finely-divided polyolefins ofrelatively high molecular weight by means of an aqueous dispersionprocess.

A further object is to prepare finely-divided synthetic organicthermoplastic resins which are substantially devoid of particles largerthan 500 microns and wherein the particles have a relatively narrow sizerange and are of spherical shape.

Another particular object is to make a finely-divided polyolefin whichis substantially devoid of particles greater than 25 microns, whereinthe average particle size is less than 10 microns, and wherein theparticles are of spherical shape.

Another object is to prepare dispersions of ethylene polymers andcopolymers which are latices.

These and other objects are accomplished by a process in which anormally solid synthetic organic polymeric thermoplastic resin issubjected to vigorous agitation in the presence of water and a 'blockcopolymer of ethylene oxide and propylene oxide as the dispersing agentat a temperature above the melting point of the resin and at a pressuresufficient to maintain the water in a liquid state until a dispersion isproduced and thereafter cooling said dispersion to below the meltingpoint of the resin.

In general, the polymers suitable for the practice of this inventioninclude any normally solid synthetic organic polymeric thermoplasticresin whose decomposition point is somewhat higher than its meltingpoint and somewhat less than the critical temperature of water. Includedare polyolefins, vinyls, olefin-vinyl copolymers, olefin-allylcopolymers, polyamides, acrylics, polystyrene, cellulosics, polyesters,and fluorcarbons.

The polyolefins most suitable for the practice of this invention includenormally solid polymers of olefins, particularly mono-alpha-olefins,which comprise from two to about six carbon atoms, e.g., polyethylene,polypropylene, polybutene, polyisobutylene, poly(4-methylpentene), andthe like. Preferred polyolefin feeds are polyethylene and polypropylene.Olefin-olefin copolymers such as ethylene copolymers with propylene,butene-l and hexene-l are also suitable. Of particular significance isthe fact that the present process is not limited to the relatively lowmolecular weight polyethylenes of prior art processes, but is equallyeffective for relatively high molecular weight polyethylene as well asfor polypropylene and other higher olefins.

Vinyl polymers suitable for use in this invention include polyvinylchloride, polyvinyl acetate, vinyl chloride/ vinyl acetate copolymers,polyvinyl alcohol, and polyvinyl acetal. Especially preferred ispolyvinyl chloride.

Suitable olefin-vinyl copolymers include ethylene-vinyl acetate,ethylene-vinyl trimethyl acetate, ethylene-vinyl propionate,ethylene-vinyl isobutyrate, ethylene-vinyl alcohol, ethylene-methylacrylate, ethylene-ethyl acrylate, ethylene-ethyl methacrylate, and thelike. Especially preferred are ethylene-vinyl acetate copolymers,including particularly copolymers having vinyl acetate concentrations ofabout 20 to about 60 weight percent.

Suitable olefin-allyl copolymers include ethylene-allyl alcohol,ethylene-allyl acetate, ethylene-allyl acetone, ethylene-allyl benzene,ethylene-allyl ether, ethyleneacrolein, and the like. Ethylene-allylalcohol is especially preferred.

Preferred among the polyamides are linear superpolycarbonamide resins,commonly referred to as nylons. Such polymers can be made by theintermolecular condensation of linear diamines containing from 6 to 10carbon atoms with linear dicarboxylic acids containing from 2 to 10carbon atoms. Equally well the superpolyamides may be made fromamide-forming derivatives of these monomers such as esters, acidchlorides, amine salts, etc. Also suitable are superpolyamides made bythe intramolecular polymerization of omega-amino-acids containing 4 to12 carbon atoms and of their amide-forming derlvatives, particularly theinternal lactams. Examples of specific nylons are polyhexamethyleneadipamide, polyhexamethylene sebacamide, and polycaprolactam. Especiallypreferred are nylons having intrinsic viscosities ranging between 0.3and 3.5 dL/g. determined in m-cresol.

Acrylic resins suitable for use in this invention include polymethylmethacrylate, polyacrylonitrile, polymethyl acrylate, polyethylmethacrylate, etc. Preferred is polymethyl methacrylate.

In general, thermoplastic resins to which this invention is applicablemay have melting points as low as 30 C. For example relatively lowdensity (0.80-1.00 g./cc.) polymers of the types described above whichhave melting points in the range of about 50 C. to about 115 C. are

useful herein. Such low density polymers particularly includehomopolymers of ethylene and low density copolymers of ethylene withsuch monomers as propylene, butene-l, hexene-l, vinyl acetate, vinyltrimethyl acetate, methyl acrylate, ethyl acrylate, vinyl alcohol andallyl alcohol. Preferred resins are ethylene homopolymers havingdensities from 0.85-0.95 g./cc. and melting points from C. to C. andethylene copolymers from the above group having densities from 0.851.00g./cc. and melting points from 55 C. to 115 C.

The dispersing agents of the present invention are watersoluble blockcopolymers of ethylene oxide and propylene oxide. Preferably, they arewater-soluble block copolymers of ethylene oxide and propylene oxidehaving a molecular weight above about 3,500 and containing a majorportion by weight of ethylene oxide. Such compounds are both stable andeffective as dispersing agents for the aforementioned thermoplasticpolymers at temperatures ranging up to about 325 C. or higher, and moreparticularly at temperatures above about 0., especially at temperaturesin the range of about to 225 C. Representative of such compounds areseveral of the non-ionic surface active agents marketed by WyandotteChemicals prepared (see the Pluronic Grid Approach, vol. II, WyandotteChemicals Corp., 1957) by polymerizing ethylene oxide on the ends of apreformed polymeric base of polyoxypropylene. Both the length ormolecular weight of the polyoxypropylene base and the polyoxyethyleneend segments can be varied to yield a wide variety of products. Forexample, one of the compounds discovered as suitable for the practice ofthis invention is Pluronic =F-98 wherein a polyoxypropylene of averagemolecular weight of 2,700 is polymerized with ethylene oxide to give aproduct of molecular weight averaging about 13,500. This product may bedescribed as containing 20 weight percent of propylene oxide and 80weight percent of ethylene oxide.

Examples of other effective Pluronics include P-105 (M.W. 6,500, 50%propylene oxide, 50% ethylene oxide), F-88 (M.W. 11,250, 20% propyleneoxide, 80% ethylene oxide), F-108 (M.W. 16,250, 20% propylene oxide, 80%ethylene oxide), and -P-85 (M.W. 4,500, 50% propylene oxide, 50%ethylene oxide). These compounds, containing at least about 50 weightpercent of ethylene oxide and exhibiting a molecular weight of at leastabout 4,500, are particularly effective as dispersing agents for theaforementioned thermoplastic polymers.

These compounds are both stable and effective at temperatures ranging upto about 325 C. or higher, and more particularly at temperatures aboveabout 160 C., especially at temperatures in the range of about 1175 to225 C. The ability of these compounds to retain effectiveness asdispersing agents at high temperatures is extremely significant anduseful. As aforementioned, the dispersing or emulsifying agents known tothe art for preparing aqueous dispersions of polyethylene are chieflysoaps. Such soaps have been found to exert their emulsifying ability attemperatures ranging up to about 160 C., but to become inactive athigher temperatures. Since in many instances dispersion temperaturesappreciably higher than 160 C. are either necessary or highly desirable,these soaps are severly limited in ability.

The temperature of operation is dependent upon the melting point, meltflow properties, decomposition temperature, and desired fineness ofdispersion of the selected synthetic organic thermoplastic resin. Whilesuch resins can be dispersed at temperatures commencing with theirrespective melting points, increases in dispersion temperature beyondthe melting point and up to the decomposition of the resins aregenerally accompanied by corresponding increases in the fluidity of themolten resin. As the fluidity of the melt increases, the dispersionsgenerally tend to develop lower average particle sizes without reqmrmgincreases in agitation effort.

The flow properties of a molten polymeric resin are closely related toits molecular weight. As the molecular weight of a given type of polymeris increased, its fluidity at a given temperature tends to lessen, thatis, the polymer tends to olfer greater resistance to breakdown to smallparticles. On the other hand, the melting point of the polymer varieslittle with changes in molecular welght. Consequently, while twopolymers of the same type, e.g., low density polyethylene, but ofdifferent molecular weights may exhibit the same melting point andtherefore be dispersible commencing at the same temperature, the highermolecular weight polymer will require high dispersion temperatures forthe same agitation effort to achieve a fineness of dispersion equivalentto that of the polymer of lower molecular weight.

A convenient measure of the fluidity or flow of a thermoplastic polymeris afforded by the melt flow rate value as outlined under ASTM testmethod Dl238-57T (2160 gram load).

As aforementioned, the soap dispersing agents of the prior art arelimited to dispersions formed at temperatures below about 160 C. Atthese temperatures and using a soap as the dispersing agent, onlypolyethylenes exhibiting a melt flow rate of about 15 or higher at suchtemperatures, for example, low density polyethylenes of low molecularweight, could be dispersed to average particle sizes in the 5 to 20micron range, and then only through very severe shearing action such asrequires use of a high speed, narrow gap colloid mill or its equivalent.Soap dispersing agents did not prove effective for preparing finedispersions of relatively high molecular weight low densitypolyethylene, higher melting polyolefins such as linear polyethylene orpolypropylene, or other higher melting polymers such as nylon, polyvinylchloride, polymethyl methacrylate, and the like.

Polymers which either do not melt or which exhibit melt flow rates belowabout 15 at temperatures below 160 C. can be readily dispersed by meansof the subject novel dispersing agents to dispersions substantiallydevoid of particles larger than 500 microns and wherein the particleshave a relatively narrow size range. Where it is desired to prepare thefinest dispersion of a given polymer, the dispersion temperature shouldbe such that the resin being dispersed exhibits a melt flow rate ofgreater than 15, and, more preferably, greater than 20. Where largeraverage particle sizes are desired or acceptable, however, dispersiontemperatures may be employed, still in combination with only relativelymild agitation, at which the polymer exhibits a melt flow rateappreciably lower than 15, for example, as low as about 2.

The dispersing apparatus or device may be any device capable ofdelivering at least a moderate amount of shearing action under elevatedtemperatures and pressures to a liquid mixture. Suitable, for example,are conventional autoclaves equipped with conventional propellerstirflers. Propellers designed to impart greater shear to the mixturetend to improve the recovered yield of pulverulent polymer, but withlittle effect on the particle size and distribution of recoveredpolymer. The particle size and distribution are somewhat dependent onthe stirring rate, higher stirring speeds resulting in finer andnarrower dispersions until an optimum speed is reached above which thereis little change. The overall recovery yield of pulverulent polyolefinfrom the dispersion is dependent upon the duration of stirring. For agiven type and rate of stirring, a period of stirring exists withinwhich maximum recoverable yields of pulverulent polyolefins result.Either shorter or longer periods of shearing result in lower recoverableyields. Preferred stirring periods generally will range from about 1 to20 minutes, and preferably from about 5 to 15 minutes. It will beunderstood, however, that the stirring rates and periods will dependupon the type of equipment utilized.

While the rate and duration of agitation affect particle size anddistribution and recoverable yields of pulverulent polymer, thesevariables can be readily optimized for any given polyolefin throughsimple, routine experimentation. For example, it was found that in aconventional, twoliter stirred pressure reactor (Parr InstrumentCompany) equipped with a turbine type stirrer polyethylene exhibiting amelt flow rate of 26 at 200 C. yielded the finest dispersion at astirring rate of 8,000 to 10,000 r.p.m. for a duration of 8 to 9 minutesat 200 C. In a 1.5-gallon stainless steel reactor equipped with acurved-blade stator and a curved-blade rotor driven by a four-horsepowerair motor, the same polyethylene yielded the finest dispersion at astirring rate of 300 to 800 r.p.m. at 200 C.

In carrying out the subject process, the selected synthetic organicthermoplastic polymer is first contacted with water and the dispersingagent. It is a particularly advantageous feature of this invention thatthe dispersing agent need not be incorporated into the polymer prior tothe introduction of the water by such means as milling and the like, butmay be introduced into the dispersing apparatus simultaneously with theother ingredients or as a solution in the aqueous phase. If desired, thedispersion process may be operated in a continuous manner, in which caseit is convenient to premix the desired ratio of dispersing agent, water,and polymer, and introduce this mixture continuously to the reactorwhile continuously removing from another part of the reactor the productdispersion.

The amount of water used in relation to the polymer dispersed generallyranges from about 0.33 to 9 parts by weight of water per part ofnormally solid polymer, preferably between about 0.8 and 4 parts perpart of polymer. To prepare dispersions which are more dilute, it isusually more economical to dilute a more concentrated dispersion.Despersions containing more than about 75 percent of polymer aregenerally quite viscous and difficult to handle. To a limited extent thedispersion becomes finer as the concentration of polymer increases,other conditions being held constant.

As little as about 0.5 part by weight of dispersing agent per parts ofnormally solid polymer may be used to produce the desired dispersions,however, it is preferred to use from about 2 to 25 parts of dispersingagent per 100 parts of polymer. Larger ratios of dispersing agentexhibit no significant influence on the fineness of dispersion and tendto make subsequent removal of the surfactant from the polymer moredifficult. In the specific embodiment of this invention wherein it isdesired to prepare polymers at very fine particle size, for example,olefin homopolymers and olefin copolymers whose average particle size isbelow about 10 microns, upon feeding the ingredients to the dispersingdevice the temperature is brought to a level at which the melt flow rateof the polymer being dispersed is at least 15, and more preferably atleast 20. Generally, the temperature at which the polymers of theinvention exhibit melt fiow rates of at least 15 ranges from about C.for low molecular weight polymers, e.g., low molecular weight lowdensity polyethylene, up to the critical temperature of water for therelatively high molecular weight and highly crystalline polymers. Forthe preferred polymers, dispersion temperatures range from about to 325C For example, a polymer such as a linear polyethylene with a flow rateof 10 at C. requires a dispersion temperature of about 245 C., whereas apolymer such as a polypropylene with a melt flow rate of 7 at 230 C.requires a dispersion temperature above about 265 C. As aforementioned,the use of lower temperatures, that is, down to the melting point of thepolymer, will also yield dispersions, but of a coarser particle size.

The pressure under which the present process is carried out is soadjusted to exceed the vapor pressure of water at the operatingtemperature so as to maintain a liquid water phase. More particularly,the pressures may range from about 1 to 217 atmospheres, and preferablyfrom about 6 to 120 atmospheres. In cases where the polymer is sensitiveto air at the elevated dispersion temperatures,

an inert gas, e.g., nitrogen or helium, may be substituted for the airnormally present, and deaerated water used.

The dispersions resulting from the above process are compositionscomprising a dispersion of a normally solid synthetic organic polymericthermoplastic resin in water in the presence of a minor amount of ablock copolymer of propylene oxide and ethylene oxide. If the dispersionis capable of forming a continuous film upon removal of the aqueousphase by evaporation, it is more definitely termed a latex. The abovedispersions or latices may be utilized in coating metal or paper, inpolish formulations, in paint formulations, for textile sizing andwaterproofing, for coating fibers, etc.

In the case of dispersions which are not latices, the temperature of thedispersion may be lowered to below the melting temperature of thedispersed polymer, and the polymer separated from the aqueous phase inthe form of discrete particles by filtration, evaporation of the water,and the like. If the temperature of the subject dispersion is lowered tobelow the boiling point of Water and the pressure released, thefinely-divided polymer may be recovered by simple atmosphericfiltration. Dispersions whose number average particle size is belowabout 10 microns are relatively stable as such and thus should be brokenby dilution with added water prior to filtration. It is an outstandingfeature of this invention that the finely-divided polymer recovered,after several washings with water, has substantially no residualdispersing agent and, consequently, requires no subsequent treatment orheating step to remove or inactivate the dispersing agent residues wherethe presence of such residues would be considered undesirable. Theaqueous filtrate and washings contain substantially all of thedispersing agent originally added, in unchanged form, and thus may berecycled to act as the dispersion medium for subsequent batches ofpolymer.

Drying of the recovered finely-divided polymer yields a free-flowingpowder of fine particle size and narrow particle size distribution.Generally, all of the dispersed particles have diameters less than 500microns. By varying the composition of the subject novel dispersingagents and the ratio of polymer to water, average particle size rangingfrom about 300' microns to as low as microns or below can be obtained.Especially preferred are particles of narrow size distribution whereinthe number of average particle size is less than 20 microns, and moredesirably less than microns. Generally, as the ratio of ethylene oxideto propylene oxide is increased in the subject novel dispersing agentsand the ratio of polymer to water is increased, the average particlesize is decreased. Further and unexpectedly, the particles of thesubject process are almost perfect spheres. The spherical shapecontributes superior fiuidization characteristics, a shorter meltingtime, and improved dispersibility to the pulverulent compositions.Consequently, the finely-divided polymers of this invention are superiorin powder form for static or fluidized dip coating, spraying, dusting,and flame spraying applications as well as for preparing stabledispersions in water or some other medium for use in roller, dip, orspray coating. The relatively high molecular weight polymers of thisinvention also find use in the preparation of heat resistant coatings,in the preparation of molded or formed shapes by powder or slush moldingtechniques, and in the preparation of foams in combination withconventional blowing agents.

The aforenoted latices may be prepared within the framework of thisinvention through the use of a combination of selected polymers orcopolymers and particular dispersing conditions. Included among thepolymers and copolymers suitable for dispersion to latices are lowdensity polyethylenes having at 190 C. a melt flow rate above about3000, and particularly between about 4000 and 10,000. Also included arecopolymers of low density polyethylene and vinyl acetate wherein thevinyl acetate constitutes at least 25 percent, and preferably between 30and 40 percent of the final copolymer, and wherein the copolymersexhibit at 190 C. melt flow rates of at least 15, and preferably betweenabout 25 and 7,000. Dispersion temperatures suitable for producing theabove latices are generally above about C., and preferably range fromabout to 225 C.

The resultant latices by definition deposit continuous films when theaqueous medium is permitted to evaporate under uniform and mildconditions, such as in air at ambient temperature and atmosphericpressure. This property imparts important and useful value to thelatices which can be used for applying continuous film coatings atambient temperature to substrates such as paper, paperboard, metal foil,glass, plastic film or sheet, and the like, and for waterproofing fibersand textiles. The latices comprise extremely fine particles of narrowparticle size distribution. For example, the polyethylene latices hadnumber average particle sizes of less than 1 micron with at least 50weight percent of the particles below 4 microns, whereas theethylene-vinyl acetate latices had number average particle sizes of lessthan 3 microns with at least 50 weight percent of the particles below 10microns.

The following examples will further illustrate this invention withoutlimitation. All parts are by weight unless otherwise indicated.

The apparatus comprised a cylindrical two-liter, 4-inch diameterpressure reactor (Parr Instrument Company) equipped with a thermowell, asafety head, a stirrer bearing and shaft, and a pressure gage. Power wassupplied to the stirrer by means of a Bodine type fractional horsepowermotor having an output of 18,000 rpm. (idling). The stirring propellerwas either a conventional blade type (3 blades, 2-inch diameter) or atype comprising two curved tooth turbine-type discs (3'inch diameter).

In the following examples, three techniques were employed tocharacterize the particle characteristics of the products. One of theseinvolved sieving by means of a Roto-Top or Air-Jet sieve usingappropriate ASTM sieves. Results of sieving analyses are expressed asweight percent passing a sieve of a particular mesh size.

A second technique utilized microscopic analysis. A sample of thedispersion was diluted with water, and a drop of the diluted dispersionwas placed under the microscope between a microscopic slide and a coverslip. By means of a calibrated ocular the sizes of 100 representativeparticles, well-distributed in the microscopic field (600xmagnification) were classified into size groups 5, 5 to 10, 10 to 25, 25to 50, and 50 to 100 microns). On the basis of duplicate counts, resultsare expressed in terms of the size of the largest particle observed andthe number average particle size.

A third technique involved the use of an electronic Coulter counter.This technique was particularly useful where a more precise count wasdesired and with disper sions, such as latices, where sieving andmicroscopic counts were impractical. The Coulter counter determines thenumber and volume of particles suspended in an electrically conductiveliquid as these particles flow singularly through a small aperturehaving an immersed electrode on each side. As a particle passes throughthe aperture, it displaces electrolyte within the aperture and therebymomentarily changes the resistance between the electrodes, causing avoltage pulse of magnitude proportional to the volume of the particle.The pulses for the particles of the sample are electrically amplified,sealed, and counted. From these counts, generally made on severalmillion particles, accurate distribution curves of both number andweight percent can be established. Results are expressed in terms of thelargest and smallest particles counted, the 50 percent weight percentilesize, and the number average particle size.

Normal particle size distribution curves plotted as particle diameterversus percent (cumulative weight percent as well as cumulative numberpercent) of dispersions which were made under optimum conditions have asigmoid shape. The curve of the particle number percent is similar tothat of the particle weight percent except that it is below that of thelatter one. In a narrow particle size distribution (1 to 25 microns) thevalue of the average particle size is 60 to 80 percent of that of the 50weight percentile or very close to that of the 50 number percentile. Ina wider distribution (1 to 30 microns) it is from 20 to 40 percent ofit. Checks of sieve analysis showed good agreement with particle sizecounts obtained on the Ooulter counter.

EXAMPLE I (A) 300 parts of a polyethylene having a density of 0.915g./cc., a melt flow rate of 22 g. per min. at 190 C., and a meltingpoint of 103 C., in the form of small chips, 18 parts of a blockcopolymer of ethylene oxide and propylene oxide of a molecular weight of13,500 and containing 20 percent by weight of propylene oxide and 80percent by weight of ethylene oxide (Pluronic F98), and 273 parts ofdeaerated water were charged to the reactor. The air Was replaced bynitrogen and heat was applied until the temperature of the mixturereached 200 C. at a pressure of 235 p.s.i. Stirring was then started andcontinued at a rate of 8,000 to 10,000 r.p.m. for a period of 8 to 10minutes. Stirring was then discontinued, and the temperature of thedispersion was allowed to drop under ambient cooling to about 90 C. Theresidual pressure was then bled off, the obtained dispersion was dilutedwith an equal volume of water, and this mixture was suction-filtered ona Buchner funnel fitted with a #41 H Whatman filter paper. The residuewas washed thoroughly with water and then dried for 4 hours at 60 C. Thedried polyethylene residue comprised 300 parts of a fine, white powderhaving a melt flow rate of 22 g. per 10 min. at 190 C. A sieve analysisgave the following particle size distribution: 100 weight percentpassing 500 microns, 78 weight percent passing 53 microns. Microscopicexamination revealed all of the particles as spherical shaped with over70 percent below 10 microns in diameter. The number average particlesize was found to be 3.65 microns.

(B) Run A was repeated omitting the addition of dispersing agent andheating the mixture to 275 C. at a pressure of 975 p.s.i. Stirring wascarried out for 9 minutes at a rate of 8,000 to 10 ,000 r.p.m. duringwhich time the mixture cooled to 225 C. No dispersion was produced, allof the product being recovered as a singly stringy mass wrapped aroundthe stirrer.

EXAMPLE II (A) 300 parts of a polyethylene having a density of 0.916g./cc., a melt flow rate of 70 g. per 10 min. at 190 C., and a meltingpoint of 103 C., 27 parts of a block copolymer of ethylene oxide andpropylene oxide of 6,500 molecular weight and containing 50 weightpercent of ethylene oxide and 50 Weight percent of propylene oxide(Pluronic P-105), and 273 parts of water were charged to the dispersingapparatus. The mixture was blanketed with nitrogen, and heat was applieduntil the temperature of the mixture reached 250 C. under a pressure of640 p.s.i. Stirring was started and continued at a rate of 8,000 to10,000 r.p.m. for a period of 8 to 10 minutes to 150 C. Stirring wasstopped, and the mixture was allowed to continue cooling down to about90 C. The residual pressure was released, and the product was separatedby filtration, washed with water, and dried. 270 parts of a white powderwas recovered. A sieve analysis of the powder showed:

83 wt. percent passing 420 microns 38 Wt. percent passing 297 microns13.5 wt. percent passing 105 microns 0.3 wt. percent passing 53 micronsMicroscopic examination revealed the particles of the powder to be inthe shape of almost perfect spheres and to have a number averageparticle size of 13.8 microns. Such a mixture is highly suitable forfluidized bed processes wherein spherical particles in the size range ofabout to 500 microns are especially preferred.

(B) Run A was repeated using 27 parts of sodium stearate as thedispersing agent in place of the ethylene oxide/propylene oxide blockcopolymer. No dispersion was produced, all of the product beingrecovered as a single stringy mass wrapped around the stirrer.

EXAMPLE III A series of dispersions were prepared from medium and highdensity polyethylenes. Reaction conditions and particle characteristicsare presented in the following table:

TABLE 1 Polymer concentration: 50 percent Dispersant:

Type: Pluronic F-108 Concentration: 9.0 percent (based on polymer)Temperature: 200 0. Pressure: 236 p.s.i. Stirling:

Rate: 8,000 to 10,000 r.p.m. Time: 7 minutes A series of dispersionswere prepared from low density low molecular weight polyethylene. Thesedispersions, when coated upon a glass plaque in a layer of about 6- milsthickness, formed continuous films upon drying in air at ambienttemperature. Polymers, dispersing conditions, and particlecharacteristics are presented in the following table:

TABLE 2 Polymer:

Density, g./cc 0. 8976 0.8976 0. 9009 0.9009 Melt flow at 190 C 6, 6006, 600 3, 040 3, 040 Melting point, C 91 91 92 92 Parts 300 300 300 300Water, parts. 273 273 273 273 Dispersant:

Type (Pluronic) F-108 F-108 F-108 F-108 Parts 27 27 27 27 Temperature,C. 200 200 250 Pressure, psi 101 235 235 660 Stirring:

Rate, r.p.m.}(lO- 8-10 8-10 8-10 8-10 Time, minutes 7-10 7-10 710 7-10Product:

Largest particle microns. 39 23 35 31 50 weight percent microns 3. 8 1.5 4. 0 3. 3 Smallest particle microns 1.0 1. 0 1. 0 1. 0 Number averageparticle size, microns 0.938 0.781 0.979 0.889

EXAMPLE V A series of dispersions were prepared from ethylenevinylacetate copolymers. These dispersions, when coated upon a glass plaquein a layer of about 6-mils thickness, formed continuous films upondrying in air. Copolymer compositions, dispersing conditions, andparticle characteristics are presented in the following table. Sinceethylene-vinyl acetate copolymers containing 40 or 50 percent or higherof vinyl acetate are completely amorphous, no melting points are shownfor the fourth and fifth copolymers in the table.

TABLE 3 Copolymer:

Vinyl acetate, percent 29 33 33 40 50 Density, gJcc 0. 95 0. 95 0. 95 0.95 0. 95 Melt flow at 190 C 150 25 15 1, 000 92. 4 Melting point, C...58 62 65 arts 300 300 300 300 300 Water, parts 273 273 273 273 273Disperant:

Type (Pluronie) F108 F-108 11-108 TIP-108 13-108 Parts 27 27 27 27 54Temperature, 200 200 250 200 200 Pressure, p.s.i 235 235 650 235 235Stirring:

Rate, r.p.m. 8-10 8-10 8-10 8-10 8-10 Time, minutes 7-10 7-10 7-10 7-107-10 Product:

Largest particle microns. 31 16 34 80 10 50 weight percent microns 3. 63. 3 l0 2. O 1. 6 Smallest particle microns 1. 0 1. 0 1. 0 1. 0 1. 0Number average particle size, microns 1. 36 2. 56 2. 84 0. 70 0. 84

EXAMPLE VI 300 parts of an ethylene-ethyl acrylate copolymer containing15.1 percent of ethyl acrylate and having a density of 0.929 g./cc., amelt flow rate of 18 g. per 10 min. at 190 C., and a melting point of 85C., 27 parts .of Pluronic F408, and 273 parts of deaerated water werecharged to the reactor. The mixture was blanketed with nitrogen, andheat was applied until the mixture reached a temperature of 200 C. at235 p.s.i. Stirring was commenced and continued at a rate of 8,000 to10,000 r.p.m. for a period of 7 to 10 minutes. Stirring was stopped, thetemperature allowed to drop to about 90 C., the residual pressure bledoff, and the product recovered as in Example I, Run A. Analysis of theproduct for particle characteristics revealed the following:

Largest particle microns 34.58 50 weight percent microns 10 Smallestparticle size microns 1.5 Number average particle size, microns 4.56

Particle shape Spherical EXAMPLE VII Example VI was repeated with anethylene-methyl acrylate copolymer containing 16 percent ofmethylacrylate. This copolymer had a density of 0.93 g./cc., a melt flowrate of 7.3 g. per 10 min. at 190 C. and a melting point of 90 C.Analysis of the product for particle characteristics showed thefollowing:

Largest particle microns 34.58

50 weight percent microns 10 Smallest particle observed microns 1.5Number average particle size, microns 4.56

Particle shape Spherical EXAMPLE VIII Example VII was repeated with anethylene-allyl alcohol copolymer containing 1.0 percent of allylalcohol. This copolymer had a density of 0.92 g./cc., a melt flow rateof 500 g. per 10 min. at 190 C. and a melting point of 102 C. Particleanalysis showed the following:

Largest particle microns 23.3 50 weight percent microns 4.0

12 Smallest particle observed microns 1.0 Number average particle size,microns 2.59 Particle shape Spherical While this invention has beendisclosed and illustrated by the foregoing specific embodiments, it willbe understood that the invention is obviously subject to othermodifications and variations without departing from its broader aspects.

What is claimed is:

1. A process for preparing in finely divided form a normally solid,synthetic organic polymeric thermoplastic resin having a decompositionpoint higher than its melting point and less than the criticaltemperature of water which comprises the steps of (a) subjecting amixture of said resin in molten form and about 0.8 to 9 parts by weightof water per part of resin to vigorous agitation in the presence of fromabout 2 to 25 parts by weight per parts of resin of a water-solubleblock copolymer of ethylene oxide and propylene oxide having a molecularWeight above about 3500 and containing at least about 50% by weight ofethylene oxide in the absence of an organic solvent at a temperaturebelow the degradation temperature of said polymeric thermoplastic resinand between about and 325 C. and at a pressure between about 6 andatmospheres such that a fine dispersion is produced, and then (b)cooling said dispersion to below the softening temperature of saidresin.

2. The process of claim 1 wherein the resin is a polyolefin.

3. The process of claim 2 wherein the polyolefin is polyethylene.

4. The process of claim 1 wherein the resin is an olefin-vinylcopolymer.

5. The process of claim 4 wherein the olefin-vinyl copolymer isethylene-vinyl acetate.

6. The process of claim 1 wherein the resulting solidified finelydivided resin is recovered from the water dis persion and issubstantially devoid of particles in excess of about 25 microns in size,has an average particle size of below about 10 microns, and issubstantially spherical in shape.

7. The process of claim 1 wherein said molten resin exhibits a melt flowrate of at least about 2.

References Cited UNITED STATES PATENTS 2,947,715 8/1960 Charlet et al.260-29.6 2,995,533 8/1961 Parmer et al. 26029.6 3,006,872 10/ 1961Benedict et al. 26029.6 3,055,853 9/1962 Pickell 26029.6 3,073,7901/1963 Bosoni 26029.6 3,245,934 4/ 1966 Krzyszkowski 26029.6 3,422,0491/1969 McClain 260--29.6 3,432,483 3/1969 Peoples et al 260873 WILLIAMH. SHORT, Primary Examiner E. A. NIELSEN, Assistant Examiner US. Cl.X.R.

106169, 188; 26029.2 R, N, E, 29.6 NR, ME, XA, 75 T, 78 S, 86.7, 87.3,88.1 R, 89.5 AW 92.1, 93.5 A, 94.9 A

